The pre-surgical assessment of visual potential in eyes with occluded optical media is a challenge which often confronts the ophthalmic surgeon. Patients with corneal opacification, dense cataracts, and vitreal bleeds of the eye may exhibit reduced vision whereby estimating retinal and visual potential is at best inaccurate.
Various devices and attendant methods, such as the laser interferometer, the Potential Acuity Meter (PAM), and electro-physiological instruments, have been developed in response to this need. The former two methods, while useful in many cases, are ineffective unless a clear "window" exists in a particular opacity. The latter method yields information that must be interpreted with considerable caution because of difficulties encountered in distinguishing between responses originating from the fovea, as opposed to the surrounding retina, the effects of optical degradation on patterned stimuli, etc..
The aforementioned disadvantages do not apply to the use of hyperacuity tests of the type discussed in co-pending U.S. Application Ser. No. 028,711, filed on Mar. 20, 1987 by Jay M. Enoch, a co-inventor for herein, "Hyperacuity Testing Instrument for Evaluating Visual Function". First, performance on hyperacuity tasks has been shown to be much less affected by optical degradation produced by both simulated and real ocular opacities than is performance of resolution tasks. Indeed, no clear "window" in a media opacity is required. Second, because hyperacuity performance falls off rapidly with increasing retinal eccentricity, foveally vs. extrafoveally-based responses are difficult to confuse when they are compared.
Metamorphopsia refers to several forms of image distortion that patients may experience who exhibit certain forms of central retinopathy or choroidopathy and/or a history of retinal detachment, sub-retinal tumor or tractional lesions. To help analyze these disturbances of visual function (as well as to delineate the bounds of central scotomas and areas of distortion), the well-known Amsler Grid test is often employed.
The Amsler Grid is a square grid, usually 10 cm.times.10 cm in size, having 5 mm.times.5 mm individual "checkerboard" squares. The grid is normally held approximately 28 to 30 cm away from a patient for testing purposes. The individual squares will then subtend a visual angle closely approximating 1.degree. or less. The squares are normally formed by while lines imprinted on a black background.
In use, a patient is asked to fixate on a spot in the center of the grid and while fixating on the spot is then asked to describe how the remainder of the grid appears. The patient is asked a series of questions in order to determine if the patient perceives missing or dim areas, whether lines are straight or wavy or crooked, etc. Usually the patient is asked to point to the defective areas the patient perceives on the grid. He/she may be asked to draw any distortions seen or to outline the area of distortion on a pad.
The degree of actual distortion is difficult to evaluate from such a test and no quantification is possible. Thus, it is difficult to evaluate modest changes in such patterns. In addition, this test pattern is of little or no use in the presence of significant ocular opacities because the grid lines become difficult to resolve, visually.
A measurement of the mean value of subjective vernier or bisection alignment, called the directional bias (or constant error), can be a sensitive indicator of metamorphopsia in many cases. One so-called "two-dot vernier" psychophysical hyperacuity test has been proposed to detect and quantify retinal distortions even in the presence of ocular opacities. This test is largely based on previous hyperacuity paradigms (gap test and perimetry test) which have been successfully applied to the detection of retinal/neural disorders behind cataracts or other ocular media opacities (corneal leukomas, vitreal membranes, and bleeds) and the evaluation of central visual acuity potential in the presence of such opacities.
Quantification of changes in an Amsler grid-type pattern is a useful goal in and of itself and is valuable to measure fine changes occurring in the presence of retinal anomalies without the added complications of occluded media.